74HC138 and NOR-Gate unexpected behaviour

Hi, I have been playing around with this for a bit but I don't understand why both of the NOR-gates at the bottom have a high output. From what I can see the upper one should be high since both inputs are low (as indicated by the led bars), while the lower one should have a low output as one of its inputs is high (again as shown by the led bars). To add to this confusion the logic analyzer shows all lines to be low which contradicts the led bar and the expected behavior of the 74HC138, which should in both cases pull Q1 low while keeping all other outputs at a high state.

(I know the wiring is a mess:)

Any help on this would be much appreciated.

Ben

Hi.
Currently you have the leds connected without a resistor.
You can set a resistance to the led bars (in properties), something like 50 Ohms should work.

Hi, setting the resistance to 50 Ohms has indeed fixed it, thanks. I still don't understand what caused this problem though.

Hi, setting the resistance to 50 Ohms has indeed fixed it, thanks.

Nice to know.

I still don't understand what caused this problem though.

Because leds are driving the voltage below 2.5 V.

By the color of the wires you can see that all inputs are low.